1. Field of the Invention
This invention relates to the field of electrostatography and, more particularly, to improvements in apparatus for controlling toner replenishment.
2. Decription of Prior Art
In electrostatography, electrostatic images formed on a dielectric recording element are rendered visible via the application of pigmented, thermoplastic particles known as toner. Typically, such toner forms part of a two-component developer mix consisting of the toner particles and magnetically-attractible carrier particles to which the toner particles adhere via triboelectric forces. During the development process, the electrostatic forces associated with the latent image act to strip the toner particles from their associated carrier particles, and the partially denuded carrier particles are returned to a reservoir.
It is well known in the art to continuously monitor the toner concentration in an electrostatographic developer mix and to replenish the mixture with toner when the concentration thereof falls below a predetermined level. Such a toner concentration monitor can be easily calibrated to compensate for toner depletion from the development system regardless of cause. Its significant drawback is that it is relatively slow to respond to abrupt changes in toner depletion rate, such as occasioned by a change in the image content of the documents being printed from ones having little image information thereon, to ones having large solid or continuous tone image areas. Typically, several minutes will elapse before the toner concentration is restored to a level at which copies of a desired image density can be obtained.
It is also known in the art to continuously monitor toner depletion from an electrostatographic development station by monitoring the amount of toner applied to the recording member during development. For example, in the commonly assigned U.S. Pat. No. 3,674,353 issued to Trachtenberg, a pair of induction plates, positioned adjacent the recording member on the upstream and downstream sides of the development station, function to sense the overall charge on the recording member before and after development. The difference in charge induced on the plates by the passage of the undeveloped and developed charge patterns has been found to be an accurate measure of the quantity of toner depleted from the development station. A toner depletion signal, proportional to the difference in charge induced on the induction plates, is used to control toner replenishment.
Another method for continuously monitoring toner depletion from a development station is useful in electronic printers. The replenishing rate is adjusted in response to the number of character print signals applied to the print head. The print signals may be in character code and a statistical average take-out rate used to estimate toner depletion, or the signals may be picture elements (pixel) signals. See for example U.S. Pat. Nos. 3,529,546 and 4,413,264.
While such toner depletion monitors are quicker to respond than are toner concentration monitors, their use for controlling toner replenishment has certain disadvantages. For example, any toner depletion, aside from that caused by image development (e.g. dusting and other losses), is not sensed by such a monitor and, hence cannot be accounted for by replenishment. Nor can such a monitor detect and cure inaccuracies or defects in the toner replenishment process. In short, toner depletion monitors are difficult, at best, to calibrate for precise control of toner replenishment.